In a 2-quart jar, mix all of the ingredients until smooth. Cover loosely with cheesecloth and let stand in a warm spot. Stir every 24 hours, until bubbly and agreeably sour, usually 4-10 days.
When it is ready, store loosely covered in the fridge.
Once a week, feed your starter: Remove from the fridge and stir vigorously, then divide starter in half. Either use, throw away or give away one half. Pour the other half into a clean 2-quart jar; add 1 cup of unbleached white flour and just enough water to make consistency similar to pancake batter (usually 1/2 to 3/4 of a cup). Re-cover with the cheesecloth and allow to sit in a warm place overnight, placing back in the fridge the next morning.
Notes
To make potato water, boil potatoes in water until tender. Drain, reserving the water for your starter. Use the cooked potatoes as you wish!